The freezing of discrete portions of food or non-food materials using liquid nitrogen has been practiced on a commercial scale for several years. While a wide variety of cryogenic apparatuses have been used to accomplish the freezing, many of them can be grouped into five typical types of apparatuses: batch freezers, immersion freezers, tunnel freezers, spiral freezers, and pelletizers.
Cabinet or batch freezers use a combination of cryogen spray and fans to rapidly cool or freeze products on racks. Batch freezers are available in different sizes and are generally adopted by small to medium sized food processors when available floor space is limited. However, freezing foodstuff in a batch freezer is not a continuous process. Moreover, because the product must be placed on trays it is difficult to process liquids or generate spherical pellets which are advantageous for downstream processing, packaging and end-use.
Straight freezing tunnels freeze foodstuff in a horizontal direction. Typically, foodstuffs are loaded on a mesh conveyor belt and cryogen is sprayed onto the product. Fans are used to increase convection and improve efficiency. When processing fine solids, however, product loss can occur through the conveyor belt mesh. While semi-solids and liquids are difficult to process, the otherwise mesh belt can be modified to a solid surface material allowing it to make disc shaped pellets. Such modified belts are not able to freeze the semi-solid or liquid into spherical pellets. Liquid nitrogen losses associated with equipment cool down and steady state operation of straight freezing tunnels will vary depending on the design, materials of fabrication and size. Cleaning straight tunnel freezers can be time consuming, especially when the freezer is poorly designed or maintained.
Immersion freezers (including pelletizers) freeze food continuously in a horizontal direction using a conveyor belt to move foodstuffs through a bath of liquid nitrogen. The liquid nitrogen bath must be replenished regularly in order to consistently freeze the foodstuffs. Immersion baths are typically used for individually quick frozen (IQF) applications to partially or fully freeze small, solid foodstuffs. Although immersion freezers can make very small pellets from semi-solids and liquids, large diameter spherical pellets are difficult to generate because they are inadvertently deformed by the belt. Liquid nitrogen losses associated with equipment cool down and steady state operation will vary depending on the design, materials of fabrication and size.
One special type of immersion freezer is disclosed by U.S. Pat. No. 5,522,237. The product to be frozen falls into a “downward” leg of a U-shaped freezer filled with liquid nitrogen. An auger screw in the “upward” leg induces a flow of liquid nitrogen downwardly through the downward leg, through the bottom of the U-shape, and upwardly through the upward leg. This flow of liquid nitrogen carries along with it the product that falls to the bottom of the downward leg. Hence, the immersion freezer is co-current. The product is then directed upwardly through the upward leg by the auger screw. The auger screw is driven to provide a higher liquid level in the upward leg than the downward leg such that the combined liquid nitrogen and product at the top of the auger screw is directed onto a downward chute. The combined liquid nitrogen and product are separated by a conveyor extending below the chute that collects the product while allowing the liquid nitrogen to pass through. The liquid nitrogen passing through is collected by another downward chute feeding into the top portion of the downward leg to complete a cycle of liquid nitrogen flow. U.S. Pat. No. 5,522,237 does not disclose a way to modify the residence time of the product in the liquid nitrogen. In order to form more or less spherical product from a liquid or semi-solid fed into the immersion freezer, the height of the vessel would have to be high enough to fully freeze the falling liquid droplets before they are transported by the auger screw conveyor. Otherwise, the more or less spherical shape would be deleteriously modified by the mechanical action of the auger screw. Therefore, the freezer vessel dimensions and/or droplet diameter would have to be optimized for freezing spherical pellets. This freezer is therefore limited because a change in the desired droplet diameter or a change in the product composition (with an accompanying change in the freezing time) would no longer result in fully freezing of the droplet before its shape is deleteriously modified by the auger screw.
Mold filling freezers for shape forming, such as the CRYOLINE® PE Freezer promoted available from Linde, were designed to process semi-solid and liquid products. This continuous freezer only crusts or partially freezes the semi-solid or liquid product in a horizontal direction with the use of a pre-cooled belt configured as a mould for producing small pellets. The resultant crust-frozen or partially-frozen pellet sheets must then be fully frozen in a conventional tunnel freezer and separated into individual pellets for further processing.
Hybrid spray and immersion tunnels, such as the Crust Flow P (CFP) and Crust Flow P2 (CFP2) available from Air Liquide, have a non-stick polymer belt which travels through a small bath of liquid nitrogen coupled with a top spray to assist in freezing. The CFP and CFP2 are particularly well adapted to crust freeze sticky or sauce-covered solid products because of the non-stick belt. The Crust Flow line of freezers can produce disk shaped chips from semi-solids and liquids. However, it is impossible to produce spherical pellets with the current Crust Flow line of freezers because the product freezes flat on the belt. While the CFP and CFP2 are very efficient freezers in terms of liquid nitrogen consumption, these freezers have a non-insignificant footprint because of their length (about 4.2 m) and because they freeze in a horizontal direction. Moreover, these freezers are designed to crust freeze up to 2500 kg/hr of product and thus may be overdesigned for smaller food processors.
Vibratory immersion freezers, such as the Crust Flow V available from Air Liquide, is a small immersion freezer (1 to 1.2 m wide and 1.7 to 2.3 m long) that uses a small vibrating bath of liquid nitrogen to transport product from one end of the freezer to the other. The Crust Flow V is particularly well adapted to crust-freeze small IQF products (e.g., cherries, cheese, cubes, and lemon juice). However, its ability to process liquids is limited due to the depth of the vibrating bath. The product residence time is also fairly short due of the length which limits the size or volume of frozen particles. Finally, the vibratory action of the bath can result in product breakage.
Another particular immersion freezer is the ALIGAL CC available from Air Liquide. This freezer is a horizontal immersion tunnel which uses a continuous current of liquid to transport the product to an exit conveyor within 10 s to 15 s. The liquid nitrogen current is generated by a paddle wheel which transports the product at a speed comparable to that of a traditional conveyor belt. The ALIGAL CC can process light solids, semi-solids and liquids because only liquid nitrogen is used to transport the product. It is also possible to freeze spherical liquid pellets due to the absence of a conveyor in the freezing zone (other than the small portion of the conveyor used to withdraw the frozen product). However, relatively larger solids are typically not processed in the ALIGAL CC since larger solids will tend to sink to the bottom of the liquid nitrogen bath. The ALIGAL CC is over 20 ft (6.1 m) long and is designed to process 1 to 4 tons/hr. This particular equipment is well suited for established food processors because of the large product throughput required to make this equipment cost effective. Making the design of the freezer more attractive for smaller food processors cannot be achieved by simply reducing both the width and length (hence, reducing the footprint). Regardless of product throughput, a certain residence time in the liquid nitrogen is required to freeze a specific product. Thus, there is a limit to which the footprint may be reduced through adjustment of the liquid nitrogen flow rate, the dimensions of the freezer (width mostly), and the exit conveyor speed.
Thus, there is a need for a freezer which has a relatively smaller footprint attractive to relatively smaller food processors that can freeze products, especially liquid or semi-solid product into more or less spherically shaped pellets, with the optional capability of operating on a continuous or near-continuous basis.